Conventionally, a projection exposure apparatus that projects a mask pattern onto a wafer is constructed so that the optical axis of the illumination optical system and the optical axis of the projection optical system coincide. Initially, unevenness in the transmittance of the projection optical system and inclinations of the principal rays at the mask surface arise rotationally symmetric to the optical axis, at least from the standpoint of design. In addition, the illumination intensity distribution of the illumination optical system and the inclinations of the principal rays are also rotationally symmetric with respect to the optical axis. Accordingly, to achieve a high degree of exposure uniformity and spatial coherence uniformity, the optical axes of the projection optical system and the illumination optical system are generally made to coincide.
Projection optical systems that use a concave mirror have been under development in recent years for use in projection exposure apparatuses. Such systems allow for a reduced number of lenses. With reference to FIG. 1, there is shown a prior art projection optical system 10 comprising, in order along a folded optical axis AZ, a mask M having a pattern thereon (not shown), three lenses 14, 16, 18, a concave mirror 20, a first fold mirror MI, a fourth lens 24, a second fold mirror M2, an aperture stop AS, a fifth lens 26, and a wafer W.
A light beam L from an illumination optical system (not shown) is incident mask M. Light beam L emerging from mask M is transmitted through lenses 14, 16, 18 and is incident concave mirror 20. Light beam L reflected by concave mirror 20 is transmitted again through lens 18 and is incident mirror Ml. Light beam L reflected by mirror MI is transmitted through lens 24 and is incident mirror M2. Light beam L reflected by mirror M2 is transmitted through aperture stop AS and lens 26, and projects an image of the mask pattern onto wafer W. Light beam L incident concave mirror 20 and the light beam reflected therefrom must be separated in a projection optical system that uses a concave mirror. Accordingly, optical axis AZ cannot constitute part of the exposure region. With reference to FIG. 2, exposure region ER is shifted from optical axis AZ of projection optical system 10, and is only a portion of the entire possible exposure region 32.
In such prior art projection exposure apparatus having a projection optical system that uses a concave mirror, such as system 10, a comparatively high exposure uniformity and spatial coherence uniformity are easy to achieve from the viewpoint of design if the optical axes of the projection optical system and the illumination optical system are made to coincide. With reference now to FIG. 3, since the possible illumination region 36 of the illumination optical system is centered on optical axis AZ, illumination efficiency is extremely low. Thus, the entire possible illumination region 36 is extremely large compared with illumination region IR.
To increase illumination efficiency, the optical axis of the illumination optical system should be made to coincide with the center of illumination region IR. In other words, the optical axis AZ of projection optical system 10 and the optical axis of the illumination optical system should be shifted. However, as discussed earlier, this is difficult from the viewpoint of design.
The necessity to make the optical axes of the illumination optical system and the projection optical system coincide is now explained in greater detail.
With reference now to FIG. 4 and projection optical system 40 with principal rays B1, spherical aberration exists at the entrance pupil EP. Consequently, even if principal rays B1 are extended at mask M, they do not meet at one point, as can be seen by the broken lines in the figure. This type of spherical aberration is called pupil spherical aberration.
With reference now to FIG. 5, if projection optical system 40 is made telecentric on the mask M side, all of principal rays B I are not parallel to optical axis AZ due to the effect of pupil spherical aberration. Moreover, the degree of inclination from parallelism with optical axis AZ differs depending on the image height h.
With reference now to FIG. 6, projection optical system 40 used in combination with an illumination optical system 44 with an optical axis AI, an entrance pupil EP' and a condenser lens 46 to form a projection exposure apparatus. Because a high degree of spatial coherence uniformity is required in a projection exposure apparatus, exit pupil EP' must be such that for all image heights, h' an image of entrance pupil EP' is formed at the center of entrance pupil EP of projection optical system 40. Therefore, the exit pupils EP and EP' must be conjugate and also aberration free (e.g., no pupil spherical aberration). To satisfy these conditions, the principal ray at each image height h' of illumination optical system 44 viewed from mask M must be inclined in relation to the inclination of the principal ray of projection optical system 40.
Generally, since an optical system is designed to be rotationally symmetric about its optical axis, the inclinations of the principal rays, discussed earlier, are also set rotationally symmetric about the optical axis. Accordingly, if optical axis Al of illumination optical system 44 and optical axis AZ of projection optical system 40 do not coincide (FIG.6) then it becomes difficult, from the viewpoint of design, to make the inclination of the principal ray at each image height coincide.
If pupil spherical aberration can be corrected in the design, then optical axes AI and AZ do not necessarily need to coincide. However, it is required that projection optical system 40 be strictly conjugate with respect to mask M and wafer W, namely that it be aberration free. In addition, the correction of pupil spherical exacerbates difficulties in the design of the projection optical system. Furthermore, the manufacture of such a projection optical system is extremely difficult. As it is, the cost of the projection optical system is already high within the total cost of the projection exposure apparatus, and the above-described design issues unfortunately further increase the cost.